Pyrometric cones are small, precisely formulated ceramic pieces designed to bend and slump at a known point, and a cone chart maps cone numbers to the firing conditions that make them do it. Cones exist because a kiln thermometer alone cannot tell a potter whether a clay body or glaze has actually finished maturing.
Cones measure heat-work: the combined effect of temperature and time, not peak temperature alone. Two firings that hit the exact same peak reading on a thermocouple can still produce different results if one ramps up faster or holds at temperature longer than the other, because clay and glaze maturation depends on cumulative heat exposure, not an instantaneous number. A cone gives a physical readout of that cumulative effect: it bends when the kiln has actually delivered enough heat-work, regardless of exactly how the temperature curve got there.
Cone numbers run in a somewhat counterintuitive sequence: low-fire cones are numbered downward from around 022 toward 1 (lower number of that series means slightly hotter), then the sequence switches direction and continues upward from 1 into the mid- and high-fire range. The practical upshot is that reading a cone number requires knowing which side of that 1/01 boundary it falls on, not just comparing numbers directly.
As a general, approximate picture (not a substitute for the specific manufacturer's chart for the cone brand in use): bisque firing typically happens at a low cone, well below where the clay body would vitrify, so the ware stays porous enough to glaze evenly. Earthenware glaze firings sit at a low cone too. Stoneware and porcelain glaze firings run considerably hotter, at a mid-to-high cone, which is where those bodies actually vitrify.
| Cone | °F | °C | Typical use |
|---|---|---|---|
| 022 | 1094 | 590 | Overglaze/lusters |
| 06 | 1828 | 998 | Common bisque target |
| 04 | 1945 | 1063 | Earthenware glaze fire |
| 6 | 2232 | 1222 | Mid-fire stoneware |
| 10 | 2345 | 1285 | High-fire stoneware/porcelain |
Cone temperature does not translate directly to a firing schedule, since the same cone number bends at a noticeably higher temperature under a faster heating rate and a lower one under a slower rate; the table above uses Orton's own 108°F/hr (medium) rate specifically so the numbers are comparable to each other, not an arbitrary mix of firing speeds.
Cones are used in two distinct ways. A witness cone is placed where it can actually be seen or checked, either through a peephole during firing or by opening the kiln afterward, and its bend confirms after the fact whether the target vitrification point was actually reached. A kiln-sitter cone, by contrast, sits inside a mechanical shutoff device wired to cut power to the kiln the moment that specific cone softens and bends far enough to trip a lever, making it part of the control system rather than just a readout. Because a kiln-sitter cone only has to bend a little to trip the mechanism, while a witness cone is judged by how far it bends, the two can give slightly different practical readings even at the same nominal cone number, which is why potters commonly use both together, a kiln-sitter cone to stop the firing and a witness cone to confirm what actually happened.